The present invention relates to a directional optical waveguide coupler and power tap, and, more particularly, to a directional optical waveguide coupler and power tap wherein a portion of the light energy propagating in a first direction in a first optical waveguide is coupled into the cladding of a section of a second optical waveguide for subsequent removal therefrom and conversion into a representative electrical signal.
In future optical fiber transmission systems, it is expected that arrangements will be required for extracting signal wave information from optical waveguides for varied purposes. Arrangements for tapping signal power from an intermediate portion of an optical fiber waveguide are known. For example, in patent application Ser. No. 522,519 of S. E. Miller, now U.S. Pat. No. 3,931,518, an arrangement is disclosed wherein means for coupling core-guided modes, propagating in an optical waveguide, into cladding modes is provided in combination with a fiber tap which removes the coupled out cladding modes for subsequent conversion into a representative electrical signal. The mode coupling means illustratively takes the form of one or more corrugated plates which are pressed against the fiber to periodically deform a region of the fiber just preceding the fiber tap. Provided the spatial periodicity of the corrugations in the plates is properly selected, and the pressure exerted on the plates is sufficiently large, the periodic deformation of the fiber causes the desired mode coupling, and assures that there is sufficient power distributed in the modes of the fiber for the dielectric coupling body of the fiber to couple out. By adjusting the pressure that the plates exert against the fiber, it is possible to adjust the degree of mode coupling that takes place in the fiber. The above-mentioned Miller arrangement, however, does not prevent spurious cladding modes from reaching the energy tap, and therefore, cannot be considered a directional energy tap.
Prior art directional energy taps have generally included two separate optical waveguides positioned within an intermediate medium and arranged relatively close and substantially parallel to each other. Light propagating in a first direction in one optical waveguide is partially or fully transferred to the other optical waveguide by either the existence of a weak coupling between the two waveguides through the intermediate medium, or the application of electric or magnetic fields to change the coupling between the guides in the region of the intermediate medium. In this regard, see, for instance, U.S. Pat. Nos. 3,408,131, issued on Oct. 29, 1968 to N. S. Kapany; 3,558,213 and 3,589,794, issued on Jan. 26, 1971 and June 29, 1971, respectively to E. A. J. Marcatili; and 3,859,503, issued on Nov. 26, 1974 to L. A. Riseberg et al.
U.S. Pat. No. 3,777,149 issued to E. A. J. Marcatili on Dec. 4, 1973, discloses a signal detection and delay equalization arrangement for multimode optical fibers. In the arrangement, there is disposed adjacent to a terminated end surface of an optical fiber, means for selectively coupling propagating mode components out of the fiber at characteristic angles. The mode coupling means comprises a high index body of a dielectric material which is disposed in a coupling relationship with the fiber core. An array of photodetectors are disposed adjacent the coupling body so that the photodetectors selectively respond to each of the coupled mode components, or to groups of such components at the characteristic angles. The detectors include suitable delays for correcting delay distortion among the various modes, and are connected to a common output circuit.
Arrangements for coupling light from a source, such as, for example, a laser, an uncladded optical fiber, or a thin-film waveguide into a smaller cladded optical fiber are known. In such arrangements, a tapered coupler or junction box directs the light from the larger source into the core of the smaller cladded optical fiber. In this manner, a maximum amount of light energy is coupled into the core of the smaller fiber while avoiding the generation of undesired cladding modes. In this regard, see, for instance, U.S. Pat. Nos. 3,779,628 issued to F. P. Kapron et al. on Dec. 18, 1973 and 3,864,019 issued to G. Smolinsky et al. on Feb. 4, 1975.